Measuring current in a power supply is an important consideration in the design and implementation of modern power supplies. A current sense function may be used for fault detection and/or protection, for current-mode controlled voltage regulation, and for current control, among other uses. Over the years, a variety of systems have been used to measure current in a power supply, including, for example, discrete resistors, use of a resistance inherent to traces of printed circuit boards, use of resistance inherent to an integrated circuit lead frame, use of inductors, magnetic sensing devices including coils, transformers and Hall effect sensors, and use of a drain-source resistance of a power metal oxide semiconductor field effect transistor (MOSFET).
One of the leading systems to measure current in a power supply uses a dedicated field effect transistor (FET) known as or referred to as a “sense-FET.” Generally, a sense-FET is small a FET, separate from the main power FET, referred to herein as the “main-FET.” Generally, a sense-FET is configured to produce a voltage corresponding to the current in the main-FET. The “current sense ratio” (CSR) is a figure of merit of the implementation of the sense-FET. The current sense ratio is a ratio of current in the main-FET to current in the sense-FET, e.g., Imain/Isense. A higher current sense ratio is generally desirable, so that the range of current sensing is extended over many decades of current in the main-FET. However, increasing CSR has been a challenge due to, for example, complex interactions between sense-FET structures and main-FET structures.
Conventional approaches to design and implementation of sense-FETs have not been found to be applicable to Split Gate Charge Balanced (SGCB) trench MOSFETs. A split gate device includes multiple layers of polysilicon in the trenches with different electrical voltages, and it has a special structure and layout to establish the proper charge balance. For example, the trenches are spaced a certain distance apart to establish a charge balance, and furthermore, any active body junction in the device must be properly surrounded by polysilicon shields that establish the charge balance.